The invention is directed to a class of fluorinated multifunctional alcohols and their derivatives such as acrylates and methacrylates, collectively referred to herein as acrylates, epoxies and vinyl ethers. The acrylates, epoxies and vinyl ethers are used as crosslinkers in compositions for optical coatings and waveguide devices.
Prior art references have disclosed that fluorine-containing polymers may be used for coating applications. See, for example, A. A. Wall, Fluoropolymers, Wiley-Interscience, 1972, and T. Deisenroth, Proc. Fluorine in Coatings II, Munich, 1997. The fluorinated polymers offer unique properties such as excellent chemical and thermal stability, good weathering and humidity resistance, low surface tension, low refractive index, and low absorption in the electromagnetic spectral region from 1300 to 1610 nm. The 1300-1610 nm region of the electromagnetic spectrum is particularly useful for fiber optic telecommunication networks. For example fluoropolymers have been used to coat optical fibers as well as to fabricate optical waveguides.
It is well known in the art that actinic radiation such as UV light permits fast curing. UV curable compositions containing fluorinated monomers, oligomers and polymers have been widely reported. See, for example, U.S. Pat. Nos. 4,508,916; 4,511,209; 4,914,171; 5,024,507; 5,062,680; 5,223,593; 5,822,489; 6,133,472; European patent No. 333,464A1; and publications including J. Pacansky, Progress in Organic Coatings, 18 (1990) 79 and R. Bongiovanni, Progress in Organic Coatings, 36 (1999) 70; all of which are herein incorporated by reference. These compositions comprise fluorinated mono- or multi-functional acrylates or vinyl ethers and at least one photoinitiator.
Most of the fluorinated acrylates disclosed in the prior art contain one or two acrylate groups per molecule. However, to achieve fast cure and high crosslinking density it is desirable that multifunctional crosslinkers having at least 3 functional groups per molecule are added to the formulations. Multifunctional hydrocarbon monomers such as acrylates, vinyl ethers and epoxies have been widely used as crosslinkers but their poor solubility in highly fluorinated monomers and polymers limits their applications in fluoro-coatings. Ausimont USA of Thorofare, N.J. provides Fluorolink T and T10, which have four hydroxyl groups per molecule. However two of the four hydroxyl groups are attached to secondary carbon atoms. As is well known to those skilled in the art, due to steric hindrance, it is very difficult to fully convert the secondary hydroxyl groups to other functional groups such as acrylates, epoxies, and vinyl ethers, especially in the presence of primary hydroxyl groups. Incomplete conversion of hydroxyl groups makes it less suitable for applications requiring low moisture uptake and low optical absorption in the 1300-1600 nm wavelength regions.
Therefore there is a need for a fluorinated alcohol having at least 3 primary alcohol groups per molecule. Such alcohol is suitable to being converted to crosslinkers containing at least 3 functional groups selected from the group consisting of acrylate, epoxy and vinyl ether. The crosslinkers may be used in fluorinated optical coating and waveguide compositions.
A fluorinated multifunctional alcohol having at least 3 primary hydroxyl groups is synthesized. The alcohol is subsequently converted to multifunctional crosslinkers such as acrylates, epoxies and vinyl ethers.
One object of this invention is to provide a new fluorinated multifunctional alcohol having at least three primary hydroxyl groups.
It is another object of the invention to provide a multifunctional alcohol that is a suitable precursor to fluorinated multifunctional acrylates, epoxies, and vinyl ethers.
Another object of the invention is to provide a new fluorinated multifunctional acrylate synthesized from the fluorinated multifunctional alcohol of the present invention.
It is still another object of the invention to provide a coating composition comprising the multifunctional acrylate of the present invention.
Still another object of this invention is to provide a method for producing an optical coating comprising (I) forming a coating composition comprising at least one acrylate and at least one free radical initiator; (II) coating the composition into a film on a substrate having a substantially uniform thickness; and (III) curing the coating composition by exposure to an actinic radiation or heat, depending on the type of the initiator.
Yet still another object of this invention is to provide a waveguide device formed by patterning the coating composition of the present invention.
The fluorinated multifunctional alcohol of the present invention has at least three primary hydroxyl groups. Its structure can be described using the following formula:
RF"Parenopenst"CH2OH)n0 
wherein RF is a fluorinated mono- or multi-valent radical such as aromatic and aliphatic esters, ethers, amides, urethanes, and mixtures thereof, and no is an integer of at least 3. Preferably the fluorine content in RF is at least 20% by weight to allow for sufficient solubility of the crosslinker in a fluorinated coating composition.
In a preferred embodiment the fluorinated multifunctional alcohol of the present invention has the following formula:
(HOH2C"Parenclosest"n1R1-Rf-R2"Parenopenst"CH2OH)n1 
wherein Rf is a monomeric or polymeric perfluorinated alkylene, oxyalkylene, arylene, oxyarylene, and mixtures thereof; R1 and R2 are monomeric or polymeric multi-valent radicals such as aromatic and aliphatic esters, ethers, amides, urethanes, and mixtures thereof; and n1 is an integer of at least 2, preferably from 2 to 4. Typically Rf has a molecular weight of at least 25% of the molecular weight of the alcohol to allow for sufficient solubility of the crosslinker in a fluorinated coating composition and to reduce optical absorption in the wavelength region of 1300-1600 nm.
In one preferred embodiment the fluorinated multifunctional primary alcohol, A, is synthesized from a fluorinated diol, B, and a carboxylic compound, C, containing one hydroxy-reacting group and at least two primary alcohol groups. B and C react to form an ester. 
wherein Rf is a monomeric or polymeric perfluorinated alkylene, oxyalkylene, arylene, oxyarylene, and mixtures thereof; R3 and R4 are monomeric or polymeric divalent moieties such as alkylene, oxyalkylene, alkylene sulfide, arylene, oxyarylene, arylene sulfide, siloxane, and mixtures thereof; X stands for a hydroxyl, halide or alkoxy group, R5 is an aliphatic or aromatic moiety and n1 ranges from 2 to 4. In the above scheme R1 and R2 in A represent the following ester groups formed, respectively: 
To avoid polymerization during the reaction it is preferred that the hydroxyl groups in C be protected before the reaction and then de-protected after the reaction.
Examples of suitable fluorinated diols, B, include, but are not limited to, 1H,1H,9H, 9H-perfluoro-1,9-nonanediol, 1H,2H,3H,3H-perfluorononane-1,2-diol, 1H,1H,10H,10H-perfluoro-1,10-decanediol, 1H,1H,12H,12H-perfluoro-1,12-dodecanediol, 1H,1H,16H,16H-perfluoro-1,16-hexadecanediol, 1H 1H,8H,8H-perfluorotetraethyleneglycol, fluoro-poly(alkylene) diol, ethoxylated fluoropoly(alkylene) diols, fluoropoly(oxyalkylene) diols having the following structures:
HOCH2O(CF2O)mCH2OH 
HOCH2CF2O(CF2CF2O)nCF2CH2OH 
HOCH2CF2CF2O(CF2CF2CF2O)nCF2CF2CH2OH 
HOCH2CF2CF2CF2O(CF2CF2CF2CF2O)nCF2CF2CF2CH2OH 
HOCH2CF2O(CF2CF2O)n(CF2O)mCH2OH 
HOCH2CF2O(CF2CF2O)m(CF2CF2CF2CF2O)n(CF2CF2O)mCF2CH2OH 
and
HOCH2CF2O(CF2CF2O)m(CF2CF2CF2O)nCF2CF2O(CF2CF2CF2O)n(CF2CF2O)mCF2CH2OH 
wherein m and n are independent integers, perfluoropolyether diols and ethoxylated perfluoropolyether diols such as Fluorolink D, D10, E and E10 commnercially available from Ausimnont USA, and other variations known to those skilled in the art.
Examples of suitable compound C include but are not limited to 2,2-bis(hydroxymethyl)propionic acid, 3-hydoxyl-2,2-bis(hydroxymethyl)propionic acid, 2,2-bis(hydroxymethyl)butyric acid, 4-hydroxy-3,3-bis(hydroxymethyl)butyric acid, and 3,3-bis(hydroxymethyl)butyric acid.
In another embodiment, the fluorinated multifunctional primary alcohol is prepared from a fluorinated diol B described above and a compound D containing at least two primary alcohol groups and one halo group suitable for ether formation. Examples of suitable compounds, D, include but are not limited to 2-(bromomethyl)-2-(hydroxymethyl)-1,3-propanediol, 2-(bromomethyl)-2-methyl-1,3-propanediol, 2-(bromomethyl)-2-ethyl-1,3-propanediol, 3-(bromomethyl)-3-methyl-1,5-pentanediol, 3-(bromomethyl)-3-(hydroxyethyl)-1,5-pentanediol. To avoid polymerization during the reaction it is preferred that the hydroxyl groups in D be protected before the reaction and then de-protected after the reaction.
In still another embodiment, the fluorinated multifunctional primary alcohol is prepared from a fluorinated dicarboxylic ester E and an amine, F, containing at least two primary hydroxyl groups. E and F react to form an amide. 
wherein Rf is a monomeric or polymeric perfluorinated alkylene, oxyalkylene, arylene, oxyarylene, and mixtures thereof; R6 and R7 are selected from the group consisting of difluoromethylene and other monomeric or polymeric divalent moieties such as alkylene, oxyalkylene, alkylene sulfide, arylene, oxyarylene, arylene sulfide, siloxane and mixtures thereof; R8 and R9 are radicals such as hydrogen, alkyl, alkylene, oxyalkylene, arylene, oxyarylene, and mixtures thereof; R is an alkyl radical such as methyl, ethyl and butyl; and p and q are integers, the sum of which is at least 2.
Non-exclusive examples of the fluorinated dicarboxylic ester E include dimethyl perfluroglutarate, dimethyl perfluoroadipate, dimethyl perflurosuberate, dimethyl perfluroazelate, dimethyl hexadecaflurosebacate, dimethyl tetracosafluro-1,10-decanedioate, and Fluorolink L and L10, available from Ausimont USA of Thorofare, N.J. Examples of suitable amines, F, include but are not limited to N,N-dihydroxyethylamine, N,N-dihydroxypropylamine, N,N-dihydroxybutylamine, serinol, 2-amino-2-ethyl-1,3-propanediol, 2-amino-2-methyl-1,3-propanediol, and tris(hydroxymethyl)-aminomethane.
Although only ester, ether and amide based syntheses are described other reactions known to those skilled in the art can be used for the preparation of the fluorinated multifunctional primary alcohols.
The primary alcohol of the present invention can be readily converted to other multifunctional compounds including, but not limited to acrylates, epoxies and vinyl ethers. In one embodiment, the fluorinated alcohol A is converted into an acrylate with acryloyl chloride using a tertiary amine. Preferably a hindered tertiary amine containing at least one tertiary or quaternary carbon atom is used. The hindered amine provides several advantages, as compared to commonly used triethylamine, such as reducing the yellowness of the products and eliminating the water washing process to remove ammonium salts formed during the acryloylation reaction. Non-exclusive examples of suitable hindered amines include N,N-dimethylisopropylamine, N,N-diisopropylethylamine, triisobutylamine, Julolidine, iminodibenzyl, 2-methylpyridine, 2,6-lutidine, 2,4,6-collidine, and mixtures thereof.
The acrylate discussed above has at least four acrylate groups per molecule. Such a molecule allows for fast curing and high crosslinking density and is suitable as a crosslinker in a coating composition containing fluorinated acrylates.
Although only the acrylate derivatives of the fluorinated multifunctional alcohol are described, other derivatives such as, but not limited to, epoxies and vinyl ethers can be synthesized from the same multifunctional alcohols using the skills well known to artisans.
The invention also provides a method of producing an optical coating comprising (1) forming a coating composition including at least one acrylate of this invention and at least one free radical initiator; (2) coating the composition into a film on a substrate having a substantially uniform thickness; and (3) curing the coating composition by exposure to an actinic radiation or heat, depending on the type of the initiator.
The coating composition is formed by thoroughly mixing the multifunctional acrylate of this invention with a free radical initiator and optionally other components such as, but not limited to, other acrylates and additives. The initiator can be either a photoinitiator, which generates free radicals when exposed to sufficient actinic radiation, or a thermal initiator, which generates free radicals when heated to a sufficient temperature. A composition containing a photoinitiator is herein called a photo-curable composition. A composition containing a thermal initiator is herein called a thermo-curable composition.
The photo-curable composition contains at least one photoinitiator having a weight percentage of 0.1-12%, preferably 0.2-6.0% and more preferably 0.5-2.0%. The photoinitiator is preferably thermally inactive below about 50xc2x0 C. Examples of suitable photoinitiators include, but are not limited to, aromatic ketones, benzil ketals, benzoin, benzoin ethers, and phosphine oxides such as benzophenone, benzyl dimethyl ketal, benzoin alkyl ethers, 1-hydroxy-cyclohexyl-phenyl ketone, benzodimethyl ketal, xcex1,xcex1-dimethyloxy-xcex1-hydroxy acetophenone, 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-propan-1-one, 2-methyl-1-[4-methylthio)phenyl]-2-morpholino-propan-1-one, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, and 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and mixtures thereof. The preferred photoinitiator composition is a mixture of at least two photoinitiators with different extinction coefficients and absorption maxima. Such mixed photoinitiator composition enables high photo contrast as well as fast curing speed. Examples of such mixtures include, but are not limited to, benzodimethyl ketal with xcex1,xcex1-dimethyloxy-xcex1-hydroxy acetophenone and 2,4,6-trimethylbenzoyldiphenylphosphine oxide with xcex1,xcex1-dimethyloxy-xcex1-hydroxy acetophenone.
The thermo-curable composition contains at least one thermal initiator at a weight percentage of 0.1-12%, preferably 0.2-6.0%, more preferably 0.5-2.0%. Suitable thermal polymerization initiators nonexclusively include peroxides such as benzoyl peroxide (BPO), di(sec-butyl)peroxydicarbonate, t-butyl peroxy-2-ethylhexanoate, t-butyl peroxyisobutyrate, 1,1-di-(amylperoxy)-cyclohexane, xcex1-cumyl peroxyneodecanoate, t-amyl peroxyneodecanoate, laurolyl peroxide, dipropylperoxydicarbonate, decanoyl peroxide, cumene hydroperoxide, t-butyl cumyl peroxide, dicumyl peroxide, di-t-butyl peroxide, t-butyl hydroperoxide, di-t-butyl diperoxy-phthalate, t-amyl perbenzoate, t-butyl perbenzoate, t-butyl peroxyacetate, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, 2,5-dihydroperoxy-2,5-dimethylhexane, t-amyl hydroperoxide, ethyl-3,3-di-(t-butylperoxy)-butyrate, 2,2-di-(t-butylperoxy)-butane and 2,2-di(t-amylperoxy)propane. Other suitable thermal initiators include alkyl azo compounds wherein the alkyl group contains from 1 to 20 carbon atoms, such as 2,2-azobis-2-methylpropionitrile; and mixtures thereof.
Optional additives may be added to the thermo-curable or photo-curable composition of this invention to enhance certain properties such as thermal stability, chemical stability, coating quality, and photo contrast. Examples of the additives include, but are not limited to, surfactants, contrast enhancers, photo stabilizers, UV absorbers, antioxidants, and dyes. Examples of surfactants include fluorinated surfactants such as Fluorad from 3M of St. Paul, Minn. and polyethers such as BYK-3500 from BYK Chemie USA of Wallingford, Conn. Suitable contrast enhancers include free radical scavengers particularly photo bleachable free radical scavengers such as the nitrones reported in U.S. Pat. No. 6,162,579. Photo stabilizers include hindered amines such as Cyasorb UV3346 from Cytec Industries of West Paterson, N.J. and TINUVIN 123S from Ciba Specialty Chemicals of Tarrytown, N.Y. UV absorbers include benzotriazoles such as TINUVIN 234 from Ciba Specialty Chemicals and benzophenone derivatives such as UVINUL from BASF of Mount Olive, N.J. Antioxidants include for example hindered phenols such as Irganox 1010 from Ciba Specialty Chemicals. Dyes include methylene green, methylene blue and the like.
The resulting composition may be coated on a variety of substrates using methods well known in the art, including, but not limited to, spin coating, slot coating, dip coating, and spray coating. Suitable substrates include, but are not limited to, silicon, glass, quartz, plastic, and metal. The coating is cured by exposure to sufficient actinic radiation or heat. The polymer coating demonstrates high cross-linking density, high optical clarity, low birefringence, good thermal stability, low glass transition temperature, and good adhesion to the substrates.
The actinic radiation used can be any light in the visible and ultraviolet regions of the spectrum as well as electron beam. Preferably the actinic radiation is UV light. The UV sources, wavelengths, intensity, and exposure procedures may be varied to achieve the desired curing degree. Useful UV sources are high pressure xenon or mercury-xenon arc lamps.
This invention further provides a waveguide device formed by patterning the optical coating composition of the present invention. The coating can be patterned with a variety of methods known to those skilled in the art such as, but not limited to, reactive ion etching and photolithography. The waveguides fabricated using the fluorinated polymers exhibit low optical loss at telecom wavelengths from 1300 to 1600 nm.